1. Field of the Invention
This invention relates to a tone receiver and, more particularly, to a receiver including a detector for detecting a given frequency signal received simultaneously with a signal carrying digital data.
2. Description of the Prior Art
In a digital data communication system, whether of the point-to-point type or of the multipoint type, modulators-demodulators (modems) are used as interfaces between the Data Terminal Equipment (DTE) and the transmission path. The purpose of these modems is to convert the data to be transmitted into a signal the characteristics of which are compatible with the passband of the transmission path. Because of the high cost of the transmission path, the data from several terminals usually are sent down the transmission path through one modem and several modems may be connected to the same transmission path. Because the resulting communication network is comparatively complex, it is desirable to reduce to a minimum the duration of those time intervals during which the transmission of data is interrupted due to equipment failures.
Whenever a failure occurs, one of the first objectives is to locate as accurately as possible the network component involved. Many solutions to this problem have been proposed. One of these consists in providing each modem with a device which, in the event of failure of the modem or any one of the components with which it is associated, generates a tone of a given frequency, referred to as alarm tone, and sends it to one of the modems of the network, called master modem, where it is to be detected and identified. The alarm tone frequency is chosen so that it lies slightly outside the spectrum of the data signal sent by the modem.
At first sight the detection and the identification of the alarm tone by the master modem would seem to involve no difficulty as it might be assumed that this would be a mere matter of passing the received signal through a very narrow band filter centered at the frequency of the alarm tone and then measuring the energy of the filtered signal. However, quite apart from the cost of such a filter, this solution would be inefficient since an absolute energy measurement would be meaningless and a relative measurement would be ineffective in view of the fact that the relative level of the alarm tone with respect to the data is, in practice, essentially variable. This could result in spurious alarm tones, being erroneously detected as valid alarm tones. Such a result could have serious consequences since the network is switched to a so-called test mode whenever an alarm tone is detected, thereby inhibiting to some extent the normal transmission of data throughout the whole network. It is therefore essential that spurious alarm tones should be ignored.
It has also been proposed to pass the signal through a hard limiter before filtering it and to measure the signal energy obtained at the output of the filter. This solution, while it allows the effects of random noise (normal line noise) to be minimized, is unfortunately ineffective in a network in which digital data are transmitted over the network simultaneously with the alarm tone and in which the level of the received alarm tone may be lower than that of the data.
The failed modem can be located in any part of the network, or, in other words, at any distance from the master modem. As a result, the relative level of the alarm tone to be detected is essentially variable. The receiver of the master modem must therefore be capable of detecting a signal the level of which may range from saturation to -30 db with respect to the data.